CN118287038A

CN118287038A - Zeolite adsorbents, method for the production thereof and use thereof

Info

Publication number: CN118287038A
Application number: CN202410313384.6A
Authority: CN
Inventors: C·拉罗什; L·布维尔; J·佩雷斯-佩里特罗
Original assignee: IFP Energies Nouvelles IFPEN; Arkema France SA
Current assignee: IFP Energies Nouvelles IFPEN; Arkema France SA
Priority date: 2015-07-09
Filing date: 2016-07-08
Publication date: 2024-07-05

Abstract

The present invention relates to zeolite adsorbents, methods for their preparation and their use. The present invention relates to zeolite adsorbents based on agglomerated zeolite X crystals including barium, potassium and sodium. These adsorbents can be used in the separation of aromatic C8 isomer fractions and in particular xylenes.

Description

Zeolite adsorbents, method for the production thereof and use thereof

The application is a divisional application of patent application with the application number of 201680040332.X, the application date of 2016, 07 and 08, and the name of zeolite adsorbent, a preparation method and application thereof.

Technical Field

The present invention relates to adsorbents based on agglomerated zeolite X crystals comprising barium and potassium, processes for their preparation, and their use.

These adsorbents can be more particularly used to produce very pure para-xylene in the liquid or vapor phase from an aromatic hydrocarbon feedstock containing isomers having 8 carbon atoms.

Background

It is known in the art that adsorbents comprising crystalline aluminosilicates can be used to separate certain hydrocarbons from mixtures comprising them. In the field of separation of aromatic hydrocarbons and in particular separation of aromatic C8 isomers, it is well recognized that the use of specific cations in the cationic sites of crystalline zeolite aluminosilicates improves the selectivity of the zeolite for one of the aromatic C8 isomers. This differential adsorption within the zeolite allows for the separation of the various aromatic C8 isomers, which is used industrially to produce very pure para-xylene from aromatic hydrocarbon feedstocks containing isomers having 8 carbon atoms.

Thus, the use of zeolite adsorbents consisting of zeolite X or Y, which in addition to sodium cations also include barium, potassium or strontium ions, alone or as a mixture, to selectively absorb para-xylene in an aromatic hydrocarbon mixture in the liquid phase is well known in the art.

Patent US 3 558 730, US 3 558 732, US 3 626 020, US 3 663 638 and US 3 960 774 show that zeolite adsorbents comprising aluminosilicate based on sodium and barium or on sodium, barium and potassium Faujasite (FAU) structures are effective for separating xylenes present in an aromatic C8 fraction, including a fraction of aromatic hydrocarbons having 8 carbon atoms. The above adsorbents are preferably used as adsorbent in liquid phase processes (especially simulated countercurrent) similar to those described in patent US2 985 589 and especially applied to aromatic C8 fractions.

However, in general, the adsorption properties of zeolites on aromatic hydrocarbons having 8 carbon atoms (xylenes and ethylbenzene) vary very slightly as a function of: the size and shape of the pores and the location of cations within the structure have an effect on the electrostatic field present within the zeolite as well as on the form of accessible volumes in the pores. Other parameters, such as polarizability of cations and molecules or structural flexibility, may also have an effect. Therefore, it is extremely difficult to theoretically and accurately predict the adsorption characteristics of zeolite for aromatic hydrocarbons having 8 carbon atoms.

In order to improve the adsorption selectivity of zeolites having the faujasite structure to aromatic C8 isomers, many studies have mentioned the effects of the Si/Al ratio of the zeolite, the nature of the exchanged cations, and its water content. Similarly, predicting the extent of improvement is very difficult because these factors exert a combined effect on the adsorption characteristics of the zeolite. In particular, in the case of a zeolite based on the barium and potassium Faujasite (FAU) structure and more precisely in the case of a zeolite based on the barium and potassium X-type Faujasite (FAU) structure, it is difficult to predict the effect of the relative proportions of barium ions and potassium.

Patent FR 2 903 978 teaches that potassium ions can occupy up to 1/3 of the exchangeable sites occupied by barium and potassium, but the patent does not exemplify any adsorbent comprising potassium and does not give any teaching that makes it possible to expect the effect of potassium on adsorption selectivity.

Patents US 8 283 274 and US 8 557 028 describe adsorbents having a potassium weight content between 0.25% and 0.9% by weight (corresponding to a molar ratio of K ₂O/(BaO+K₂O+Na₂ O between 1.3% and 4.5%). Patent US 8 557 028 claims adsorbents having a potassium weight content between 0.9% and 1.5% (equivalent to a molar ratio of K ₂O/(BaO+K₂O+Na₂ O between 4.5% and 7.5%). The examples of the latter patent show improved productivity using adsorbents having potassium contents between 0.5% and 0.7% by weight (equivalent to molar ratios of K ₂O/(BaO+K₂O+Na₂ O of 2.5% and 3.5%). Patent CN 1267185 describes adsorbents having a BaO/K ₂ O molar ratio between 10 and 40 (corresponding to a K ₂O/(BaO+K₂O+Na₂ O molar ratio between 2.4% and 9.1%).

Recent patent US2015/0 105 600 describes adsorbents based on zeolite X, barium and potassium, having a molar ratio of K ₂O/(BaO+K₂O+Na₂ O between 15% and 40%.

Synthesis of zeolites results in such crystals (typically in powder form): its use on an industrial scale is particularly difficult (significant loss of raw materials during operation). Agglomerated forms of these crystals are therefore preferred, in the form of pellets (grains), yarns, and other agglomerates, which may be obtained by extrusion, granulation, and other agglomeration techniques known to those skilled in the art. These agglomerates do not have the drawbacks inherent in the powdered material.

These agglomerates, whether they are in the form of tablets, beads, extrudates or the like, are generally formed by: zeolite crystals, which constitute the active ingredient (in the sense of adsorption); and binders intended to ensure the cohesion (cohesion ) of the crystals in agglomerate form and to give them a mechanical strength sufficient to withstand the vibrations and movements to which they are subjected during the operations for separating the isomers of the C8 aromatic fraction. However, the adsorptive properties of these agglomerates are significantly reduced relative to crystalline powders due to the presence of the agglomeration binder which is inert to adsorption. In order to overcome this drawback of the agglomeration binder being inert to the adsorption properties, various means have been proposed, among which are the conversion of all or at least part of the agglomeration binder into a zeolite that is active from the adsorption point of view. This operation is now known to the person skilled in the art, for example under the name "zeolitization". In order to easily carry out this operation, a zeolitizing binder is used, which is generally a clay belonging to the kaolinite family, and is preferably calcined beforehand, generally at a temperature between 500 ℃ and 700 ℃.

Patent application FR 2 789 914 describes, for example, a process for the manufacture of zeolite X agglomerates with a Si/Al ratio between 1.15 and 1.5, comprising barium and optionally potassium. The agglomerates thus obtained have improved properties after the zeolitization of the binder, from the point of view of the adsorption of the para-xylene contained in the aromatic C8 fraction, with respect to adsorbents prepared from the same amount of zeolite X and binder but whose binder is not zeolitized.

Important factors affecting the performance of the adsorptive separation process include, among others, adsorption selectivity, adsorption capacity, and mass transfer kinetics (which define the adsorption and desorption rates of different compounds). The adsorbent must therefore have good mass transfer properties to ensure a sufficient number of theoretical (column) plates for achieving efficient separation of the mixed species, as demonstrated in the publication entitled "PRINCIPLES OF ADSOPRTION AND ADSORPTION PROCESSES", john Wiley & Sons, (1984), pages 326 and 407, ruthven. Ruthven (supra, page 243) shows that in the case of agglomerated adsorbents, the total mass transfer depends on the sum of the intra-and inter-crystalline diffusion resistances. The intra-crystalline diffusion resistance is proportional to the square of the radius of the crystal and inversely proportional to the diffusivity of the intra-crystalline molecule.

The inter-crystal diffusion resistance (also known as macropore resistance) is itself proportional to the square of the radius of the agglomerates and inversely proportional to the diffusivity of the molecules in the macropores. For a given zeolite structure, a given agglomerate size and a given operating temperature, the diffusivity is fixed and the only way to improve mass transfer is to reduce the diameter of the crystals. Thus, by reducing the diameter of the crystal, a gain in terms of total mass transfer will be obtained.

Thus, one skilled in the art would expect the following agglomerated zeolite adsorbents: when they are prepared from small zeolite crystals, in a liquid phase process (e.g., of the simulated countercurrent type) for separating para-xylene contained in an aromatic C8 fraction, they have both good xylene adsorption capacity and good selectivity to para-xylene, thus having very good xylene separation properties. However, it is not possible for a person skilled in the art to define in principle or theoretically and accurately the adsorption characteristics of FAU zeolites (in particular of the X-type) having a specific barium and potassium composition for aromatic hydrocarbons having 8 carbon atoms.

It is an object of the present invention to provide a novel adsorbent based on zeolite X comprising barium, potassium and sodium and having a specific barium, potassium and sodium composition, which is optimal for maximizing the production efficiency of the process for separating para-xylene contained in the aromatic C8 fraction. The invention also proposes a process for separating xylenes using an adsorbent based on zeolite X having a specific barium, potassium and sodium composition, which allows the production of para-xylene with high purity and improved production efficiency, starting with an aromatic hydrocarbon feedstock comprising isomers having 8 carbon atoms.

Disclosure of Invention

The present invention relates to a zeolite adsorbent comprising zeolite X crystals and comprising barium, potassium and sodium, wherein the molar ratio K ₂O/(K₂O+BaO+Na₂ O (of the species in oxide form) is between 8.0% and 8.6%, preferably between 8.2% and 8.4%. In an advantageous variant, the molar ratio K ₂O/(K₂O+BaO+Na₂ O) is equal to 8.3%. In the present invention, the molar ratio of K ₂O/(K₂O+BaO+Na₂ O) is expressed as a percentage of the sum of the number of moles of K ₂ O relative to the number of moles of (K ₂O+BaO+Na₂ O).

The content of sodium oxide Na ₂ O is advantageously less than 0.3% by weight and preferably less than 0.2% by weight relative to the total mass of the adsorbent.

The total content of alkali metal or alkaline earth metal ion oxides other than barium oxide BaO, potassium oxide K ₂ O and sodium oxide Na ₂ O is advantageously less than 1% by weight, preferably between 0 and 0.5% by weight and very preferably between 0 and 0.3% by weight, relative to the total mass of the adsorbent.

The zeolite X crystals advantageously have a Si/Al atomic ratio between 1.00 and 1.50, preferably between 1.05 and 1.50 and more preferably between 1.10 and 1.50.

The zeolite X crystals have a number average diameter of less than or equal to 1.5 μm, preferably between 0.1 μm and 1.2 μm, more preferably between 0.1 μm and 1.0 μm.

The loss on ignition of the adsorbents according to the invention, measured at 950 ℃, according to standard NF EN 196-2, is advantageously between 4.0% and 7.7%, preferably between 4.5% and 6.5% and very preferably between 4.8% and 6% by weight.

The number average diameter of the adsorbents according to the invention may be between 0.2mm and 2mm, in particular between 0.2mm and 0.8mm and preferably between 0.2mm and 0.65 mm.

The invention also relates to a process for preparing an adsorbent as described above, comprising at least the steps of:

a) Agglomerating a powder of zeolite X crystals with a binder and shaping, followed by drying and calcining,

B) Optionally the binder is zeolitized,

C) The agglomerates are cation exchanged by contact with barium ions, or potassium ions, or a solution of barium and potassium ions,

D) Cation exchange with potassium when the exchange step c) is carried out with a solution of barium ions alone or with barium when the exchange step c) is carried out with a solution of potassium ions alone,

E) The product thus treated is then washed and dried,

F) The zeolite adsorbent thus obtained is activated.

Preferably, the process for preparing the adsorbent involves a step b) of zeolitizing the binder.

Preferably, the barium ion, or potassium ion, or the solution of barium and potassium ions of steps c) and d) has a concentration between 0.2M and 2M.

The invention also relates to an adsorbent as described previously, obtainable according to the above preparation process.

The invention also relates to the use of the adsorbent according to the invention in a process for:

separation of aromatic C8-isomer fractions and in particular xylenes,

Separation of substituted toluene isomers such as nitrotoluene, diethyltoluene, toluenediamine and the like,

The separation of the cresols,

The separation of the polyol from the polyol,

And in particular para-xylene from aromatic isomer fractions having 8 carbon atoms.

The invention also relates to a process for recovering para-xylene in the liquid phase from an aromatic hydrocarbon isomer fraction containing 8 carbon atoms by: the use of the adsorbent according to the invention adsorbs para-xylene in the presence of a desorbent, preferably selected from toluene and para-diethylbenzene.

The process may be a simulated moving bed, preferably using simulated countercurrent flow.

The invention also relates to a process for recovering para-xylene in the gas phase from an isomer fraction of aromatic hydrocarbons containing 8 carbon atoms by: the use of the adsorbent according to the invention adsorbs para-xylene in the presence of a desorbent, preferably selected from toluene and para-diethylbenzene.

The invention also relates to a process for separating polyols using the adsorbent according to the invention.

The invention also relates to a process for separating substituted toluene isomers such as nitrotoluene, diethyltoluene and toluenediamine using the adsorbents according to the invention.

Finally, the invention relates to a process for separating cresols using the adsorbents according to the invention.

In the following text, and unless otherwise indicated, the limits of the ranges of values are included in the ranges, particularly in the expressions "between …" and "range from … to.

Detailed Description

Thus, a first object of the present invention is a zeolite adsorbent based on zeolite X. These adsorbents are particularly suitable for use in processes for separating para-xylene in the liquid phase, preferably in simulated countercurrent flow.

The present invention thus relates to a zeolite adsorbent comprising zeolite X crystals and comprising barium, potassium and sodium, wherein the molar ratio K ₂O/(K₂O+BaO+Na₂ O) is between 8.0% and 8.6%, preferably between 8.2% and 8.4%, and more preferably equal to 8.3%.

The adsorbent according to the invention may also comprise a non-zeolitic phase, i.e. an amorphous phase which is substantially inert to adsorption. In the case where the adsorbent according to the invention comprises a non-zeolitic phase, said K ₂O/(K₂O+BaO+Na₂ O) molar ratio takes into account the oxides comprised in said non-zeolitic phase.

The content of sodium oxide Na ₂ O in the adsorbent according to the invention is advantageously less than 0.3% by weight and preferably less than 0.2% by weight, relative to the total mass of the adsorbent.

The total content of alkali metal or alkaline earth metal ion oxides other than barium oxide BaO, potassium oxide K ₂ O and sodium oxide Na ₂ O in the adsorbent according to the invention is advantageously less than 1% by weight, preferably between 0 and 0.5% by weight and very preferably between 0 and 0.3% by weight, relative to the total mass of the adsorbent.

The zeolite adsorbent according to the present invention is an adsorbent based on zeolite crystals of type X FAU. The term "zeolite X" means a zeolite having a Si/Al atomic ratio between 1.00 and 1.50, preferably between 1.05 and 1.50, and more preferably between 1.10 and 1.50.

Among zeolite X, two subgroups, called zeolite LSX and zeolite MSX, are now widely accepted. Zeolite LSX has a Si/Al atomic ratio equal to about 1 and zeolite MSX has a Si/Al atomic ratio between about 1.05 and about 1.15.

In the zeolite adsorbent of the present invention, and according to a preferred embodiment, the term "FAU zeolite of type X" means FAU zeolite of type X defined above, these being classified porous (HIERARCHICALLY POROUS), i.e. classified porous zeolite of type X (or zeolite HPX), classified porous zeolite of type MSX (or HPMSX) and classified porous zeolite of type LSX (or HPLSX), and more particularly FAU zeolite having a Si/Al atomic ratio of between 1.00 and 1.50, preferably between 1.05 and 1.50, more preferably between 1.05 and 1.40 and even more preferably between 1.15 and 1.40.

The invention also includes a zeolite adsorbent comprising a mixture of two or more FAU zeolites of hierarchical porosity as just defined.

The term "hierarchically porous zeolite" means a zeolite having both micropores and mesopores, in other words, a zeolite that is both microporous and mesoporous. The term "mesoporous zeolite" means a zeolite that: the microporous zeolite crystals thereof have nano-sized internal voids (mesopores) concomitant with microporosity, which are readily identifiable by Transmission Electron Microscopy (TEM), as described, for example, in US 7 785 563: observation by Transmission Electron Microscopy (TEM) allows inspection of whether the zeolite crystals are filled zeolite crystals (i.e., not mesoporous), or aggregates of filled zeolite crystals, or mesoporous crystals, or aggregates of mesoporous crystals.

The crystalline structure of the X-type FAU zeolite in the zeolite adsorbent of the present invention is confirmed by X-ray diffraction (known to those skilled in the art by XRD abbreviation).

According to a preferred embodiment, the zeolite adsorbent has a Si/Al atomic ratio between 1.00 and 2.00, preferably between 1.00 and 1.80, more preferably between 1.15 and 1.80, and even more preferably between 1.15 and 1.60.

In this document, the term "number average diameter" or "size" is used with respect to zeolite crystals and with respect to zeolite agglomerates. The method for measuring these magnitudes is explained later in the specification. According to a preferred embodiment of the invention, the zeolite X crystals have a number average diameter of less than or equal to 1.5 μm, preferably between 0.1 μm and 1.2 μm and more preferably between 0.1 μm and 1.0 μm.

The zeolite adsorbent of the present invention is preferably in the form of agglomerates, i.e. it is composed of zeolite crystals and at least one non-zeolite phase which may include an agglomeration binder which allows agglomeration of said crystals. Thus, the zeolite adsorbents of the present invention are often referred to as "agglomerates" in this specification.

The mass fraction of zeolite X in the adsorbent according to the invention may be at least 80% by weight, preferably at least 90% by weight, relative to the total weight of the adsorbent, which may be up to 100% by weight and typically up to 99.5% by weight.

According to a preferred embodiment, the zeolite adsorbent according to the invention has a loss on ignition measured at 950 ℃ according to standard NF EN 196-2 of between 4.0% and 7.7%, preferably between 4.5% and 6.5% and advantageously between 4.8% and 6%.

The zeolite adsorbents according to the present invention preferably have a mechanical strength generally greater than or equal to 1.8MPa, typically greater than or equal to 2.1 MPa. The mechanical strength was measured via Shell method series SMS1471-74 modified for agglomerates smaller than 1.6mm in size.

The adsorption capacity itself is measured by: the micropore volume of the adsorbent was measured and evaluated by the Dubinin-Raduskevitch equation by nitrogen (N ₂) adsorption at a temperature of 77K after degassing at 300 ℃ for 16 hours under vacuum. The micropore volume of the zeolite adsorbents of the present invention is thus measured to be greater than 0.250cm ³/g, typically in the range of 0.256cm ³/g-0.288cm³/g.

According to a further aspect, the present invention relates to a process for preparing zeolite agglomerates as just defined, comprising at least the following steps:

a) Agglomerating zeolite X crystals (powder) with a binder and shaping the agglomerates, followed by drying and calcining,

B) Optionally zeolitizing the binder, preferably via the action of an alkaline solution,

C) Cation exchange the agglomerates by contact with barium ions, or potassium ions, or a solution of barium and potassium ions,

E) The product thus treated is then washed and dried,

F) Activating the zeolite agglomerates.

Preferably, the process for preparing zeolite agglomerates involves a step b) of zeolitizing the binder.

The size of the zeolite X crystals used in step a) is measured by observation with a Scanning Electron Microscope (SEM) or by observation with a Transmission Electron Microscope (TEM). The SEM or TEM observations also allow confirmation of the presence of non-zeolitic phases in the agglomerates, including for example the binder or residual binder or any other amorphous phase that is not converted during the optional zeolitization step.

According to one embodiment, zeolite X used in step a) comprises, and preferably is, a classified porous zeolite X type FAU. Crystals of the classified porous X-type FAU zeolite having a large external surface area can be obtained according to various methods known to those skilled in the art, for example, according to the synthesis described by Inayat et al (angelw.chem.int.ed., (2012), 51, 1962-1965).

The crystals may also be prepared by seeding and/or by adjusting synthesis operating conditions such as SiO ₂/Al₂O₃ ratio, sodium content and basicity of the synthesis mixture or alternatively according to conventional procedures known to those skilled in the art for post-treatment of zeolite crystals of type X FAU.

The post-treatment process generally consists in removing atoms from the zeolite network that has been formed as follows: via one or more acid treatments to dealuminate the solids, these treatments are followed by one or more washes with sodium hydroxide (NaOH) to remove the formed aluminum-containing residues, as described, for example, by d.verboekend et al (adv.funct.mater., 22, (2012), pages 916-928); or alternatively via a treatment combining the action of an acid with the action of a structuring agent improving the efficacy of the acid treatment, as described for example in patent application WO 2013/106816.

Agglomeration and shaping (step a) may be carried out according to any technique known to the person skilled in the art, such as extrusion, compaction, agglomeration, etc. The proportions of agglomeration binder and zeolite used, optionally zeolitizable (see definition below), are typically those of the prior art, i.e. from 5to 20 parts by weight of binder per 95 to 80 parts by weight of zeolite. The agglomerates resulting from step a), whether in the form of beads, extrudates, etc., generally have a number average diameter (or their largest dimension when they are not spherical) of between 0.2mm and 2mm, in particular between 0.2mm and 0.8mm and preferably between 0.2mm and 0.65 mm.

At the end of step a), the finest agglomerated particles can be removed by cyclone separation and/or sieving and the coarsest particles (for example in the case of extrudates) can be removed by sieving or crushing.

The agglomeration binder used in step a) may be zeolitizable. It then comprises at least 80%, preferably at least 90%, more preferably at least 95%, more particularly at least 96% by weight of a zeolitizable clay and may also comprise other mineral binders such as bentonite, attapulgite and the like. The term "zeolitizable clay" means a clay or clay mixture that is convertible (typically via the action of an alkaline solution) to a zeolite material (i.e., a material that is active in the adsorption sense). The zeolitizable clay generally belongs to the families of kaolin, kaolinite, nacrite, dickite, halloysite and/or metakaolin. Kaolin is preferred and most commonly used.

Other clays such as sepiolite or attapulgite, among others, may also be used.

In all cases, the clays can be used in their original form or can have been previously subjected to one or more treatments selected from calcination, acid treatment, chemical modification and the like.

The zeolite X powder used in step a) may result from the synthesis of zeolite X crystals, such as zeolite NaX, comprising mainly or even exclusively sodium cations, but the use of the following powders does not constitute a departure from the context of the present invention: after its synthesis and before its use in step a), it has undergone one or more cation exchanges.

During step a), one or more additives may be added in addition to the zeolite X powder and the binder, for example additives intended to promote agglomeration or to improve the hardening of the agglomerates formed, such as lignin, starch, carboxymethyl cellulose, and other additives known to the person skilled in the art. Silica may also be added. The optional silica source may be any type known to those skilled in the art as an expert in zeolite synthesis such as colloidal silica, diatomaceous earth, perlite, fly ash, sand, or any other form of solid silica.

After drying in step a), calcination is carried out at a temperature typically between 500 ℃ and 600 ℃. In the case where the shaping is performed with a zeolitizing clay, this step makes it possible to convert the zeolitizing clay (typically kaolin) into metakaolin, which can then be converted into zeolite during the zeolitizing step (step b)). The principle is presented in "Zeolite Molecular Sieves", D.W. Breck, john Wiley and Sons, new York, (1973), pages 314 to 315.

The zeolitization of the binder for agglomeration is carried out according to any method known to the person skilled in the art and can be carried out, for example, by immersing the product from step a) in an alkaline base solution, generally an aqueous solution, for example an aqueous solution of sodium hydroxide and/or potassium hydroxide.

As a general rule, the concentration of the basic zeolitizing solution is preferably between 0.5M and 5M. The zeolitization is preferably carried out thermally, at temperatures above room temperature, and typically at temperatures between about 80 ℃ and 100 ℃, for example between room temperature (i.e., about 20 ℃) and the boiling point of the alkaline zeolitizing solution. The duration of the zeolitization process is generally between tens of minutes and hours, preferably between about 1 hour and 8 hours.

Steps c) and d) of exchanging the cations of zeolite X with barium and/or potassium are according to standard methods known to the person skilled in the art and are generally carried out by: contacting the agglomerates from step a) or step b) with a salt in an aqueous solution, for example barium chloride (BaCl ₂) for barium exchange and/or potassium chloride (KCl) for potassium exchange, at a temperature between room temperature and 100 ℃, and preferably between 80 ℃ and 100 ℃. In order to obtain a low sodium oxide content, i.e. a content below 1%, it is preferred to operate with a large excess, typically about 10-12 times excess, of barium and/or potassium ions relative to the zeolite cations desired for exchange, advantageously by performing successive exchanges.

To achieve a molar ratio of K ₂O/(K₂O+BaO+Na₂ O between 8.0% and 8.6%, one or more ion exchanges are carried out by using an aqueous solution of barium ions and an aqueous solution of potassium ions (for example barium chloride and potassium chloride) at a concentration typically between 0.05M and 1.5M, preferably between 0.1M and 1.2M.

According to one embodiment, at least one exchange is carried out by using an aqueous solution of barium ions and potassium ions (corresponding to step c)). In this embodiment, step d) of the method according to the application is not carried out. This embodiment is the preferred embodiment.

According to another embodiment, at least one ion exchange is first carried out by using an aqueous solution of barium ions (corresponding to step c)), and then at least one ion exchange is carried out by using an aqueous solution of potassium ions (corresponding to step d)).

According to another embodiment, at least one ion exchange is performed by using an aqueous potassium ion solution (corresponding to step c)), and subsequently at least one ion exchange is performed by using an aqueous potassium ion solution (corresponding to step d)).

Each exchange step may be performed one or more times.

The exchange is carried out according to standard methods known to the person skilled in the art, for example between room temperature (20 ℃) and 100 ℃, preferably between 80 ℃ and 100 ℃, usually at atmospheric pressure. The duration of the exchange step is generally between a few minutes and a few hours, preferably between about 30 minutes and 3 hours.

K ₂O/(K₂O+BaO+Na₂ O) is achieved according to standard methods known to the person skilled in the art and, for example, by exchanging with a large excess of barium ions at a time to rapidly obtain a low content of sodium oxide, i.e. less than 1%, preferably less than 0.3%, followed by another exchange of aqueous potassium ion solution due to the necessary molar amount of potassium ions to achieve the target K ₂O/(K₂O+BaO+Na₂ O).

As indicated previously, it is also possible to agglomerate in step a) the zeolite X powder already containing potassium ions (prior to step a), the cations present in the starting zeolite X, typically sodium cations, being pre-exchanged with potassium ions) and optionally to omit the potassium exchange during steps c) and/or d).

Washing is then carried out, usually and preferably with water, followed by drying of the agglomerates thus obtained.

Activation after drying is conventionally carried out according to methods known to the person skilled in the art, for example at temperatures generally between 100 ℃ and 400 ℃, preferably between 200 ℃ and 300 ℃. The purpose of this activation step f) is to fix the water content of the adsorbent and the loss on ignition in a manner optimal for the intended use. The process is generally carried out by thermal activation, preferably at between 200 ℃ and 300 ℃ for a time determined as a function of the desired water content and the desired loss on ignition, typically 1-6 hours.

The invention also relates to the use of the above zeolite adsorbent as an adsorption agent capable of advantageously replacing what is described in the literature for:

separation of aromatic C8 isomer fractions and in particular xylenes,

Separation of substituted toluene isomers such as nitrotoluene, diethyltoluene, toluenediamine, and the like,

The separation of the cresols,

Separation of polyols such as sugars.

The invention relates in particular to a process for recovering para-xylene in high purity from an aromatic isomer fraction having 8 carbon atoms, carried out in a liquid phase process as well as in a gas phase process, which consists in using the zeolite adsorbent according to the invention as para-xylene adsorbent reagent. The term "high purity para-xylene" means a product suitable for use in the manufacture of terephthalic acid or dimethyl terephthalate, i.e. a purity of at least 99.5% by weight, preferably at least 99.7% by weight, preferably at least 99.8% by weight and even more preferably at least 99.9% by weight. The purity of paraxylene can be determined via chromatography. Gas chromatography, which can be used to determine both the purity of paraxylene and the specific amount of impurities, is the ASTM D-3798 method.

Thus, by preparative adsorption liquid chromatography (batch (batchwise)) and advantageously continuously on a simulated moving bed, i.e. using simulated countercurrent or simulated concurrent flow, and more particularly using simulated countercurrent flow, the desired product (para-xylene) can be isolated.

The process for recovering para-xylene according to the present invention using the adsorbent described according to the present invention has the advantage of maximizing the productivity, even if the transport rate (DELIVERY RATE) of the feedstock to be treated is maximized. This is particularly true for commercial adsorption units that simulate a countercurrent type, under the following operating conditions:

Number of beds: 6-30,

Number of zones: at least 4 operating zones, each located between the feed point and the take-off point,

Temperature: at 100 c and 250 c, preferably between 150 c and 190 c,

Pressure of the industrial unit: between bubble pressure (bubble pressure) of xylene at process temperature and 3MPa,

Desorbent/feed transfer rate ratio: between 0.7 and 2.5 for individual adsorption units, for example between 0.9 and 1.8 and between 0.7 and 1.4 for adsorption units combined with crystallization units,

Recycle ratio (i.e., the ratio of the average recycle transport rate (average of transport rates of zones weighted by the number of beds per zone) to feed transport rate): between 2.5 and 12, preferably between 3.5 and 6.

Reference may be made in this respect to the teachings of patents US2 985 589, US 5 284 992 and US 5 629 467.

The operating conditions of the simulated forward flow industrial adsorption unit are generally (substantially) the same as those of the unit operating in simulated countercurrent, except for the recycle ratio (which is typically between 0.8 and 7). Reference may be made in this respect to patents US 4 402 832 and US 4 498 991.

The desorbent solvent may be any desorbent known to those skilled in the art and having a boiling point lower than the boiling point of the feedstock, such as toluene, and a desorbent having a boiling point higher than the boiling point of the feedstock, such as p-diethylbenzene (PDEB). The selectivity of the adsorbents according to the invention for the adsorption of para-xylene contained in the aromatic C8 fraction is optimal when their loss on ignition measured at 950 ℃ is generally between 4.0% and 7.7%, preferably between 4.5% and 6.5% and very preferably between 4.8% and 6.0%.

Characterization technique

Particle size of the crystals:

The number average diameter of the zeolite X crystals and the zeolite X crystals contained in the agglomerates used in step a) is evaluated by observation with a Scanning Electron Microscope (SEM) or by observation with a Transmission Electron Microscope (TEM).

To evaluate the size of the zeolite crystals on the sample, a set of images was taken at a magnification of at least 5000. The diameter of at least 200 crystals is then measured using proprietary software, such as SMILE VIEW software from publisher LoGraMi. The accuracy is about 3%.

Chemical analysis of zeolite adsorbent-Si/Al and K ₂O/(K₂O+BaO+Na₂ O) ratio:

The chemical elemental analysis of the final product obtained at the end of the previously described steps a) to f) can be carried out according to various analytical techniques known to the person skilled in the art. Among these techniques, mention may be made of chemical analysis techniques by X-ray fluorescence as described in the standard NF EN ISO 12677:2011 on a wavelength-dispersive spectrometer (WDXRF), for example Tiger S8 from Bruker.

X-ray fluorescence is a non-destructive spectroscopic technique that uses photoluminescence of atoms in the X-ray region to determine the elemental composition of a sample. Excitation of atoms (typically by an X-ray beam or by bombardment with electrons) produces specific radiation after returning to the ground state of the atoms. X-ray fluorescence spectroscopy has the advantage of being less dependent on the chemical combination of elements, which provides accurate quantitative and qualitative determinations. For each oxide, a measurement uncertainty of less than 0.4 wt% is conventionally obtained after calibration. In the present invention, the barium, silicon and aluminum contents are preferably measured by the X-ray fluorescence method described above.

On the other hand, for lighter elements in terms of atomic weight, such as sodium or potassium, present in the adsorbent, inductively coupled plasma-optical emission spectrometry (ICP-OES) according to the standard UOP 961-12 is preferred for greater accuracy.

ICP is a method of atomic emission spectrometry analysis by which a source is plasma generated by inductive coupling. The method is also commonly used to determine the content of various elements such as silicon, aluminum, potassium, sodium and barium. In the present invention, the sodium content and potassium content are preferably measured via the ICP method according to standard UOP 961-12. In this case, sodium is obtained with a measurement uncertainty of less than 0.01% for the weight content of sodium oxide in the adsorbent and less than 0.02% for potassium for the weight content of potassium oxide in the adsorbent.

These elemental chemical analyses allow to check both the Si/Al atomic ratio of the zeolite in the agglomerate and the quality of the ion exchange described in step c) and in step d). In the description of the present invention, the measurement uncertainty of the Si/Al atomic ratio was 0.05.

The quality of the ion exchange is related to the number of moles of sodium oxide Na ₂ O remaining in the zeolite agglomerates after the exchange. More precisely, the degree of ion exchange with barium is determined by the ratio between the moles of barium oxide BaO and the moles of the combination (bao+k ₂O+Na₂ O). Similarly, the extent of ion exchange with potassium is determined by the ratio between the moles of potassium oxide K ₂ O and the moles of the combination (bao+k ₂O+Na₂ O). BaO, K ₂ O and Na ₂ O are expressed in oxide form. The total degree of ion exchange with barium and potassium was evaluated from the sum of the two degrees of exchange described previously, which corresponds to the ratio between the sum of the moles of barium oxide and potassium oxide (bao+k ₂ O) and the moles of the combination (bao+k ₂O+Na₂ O). It should be noted that the content of the various oxides is given as a weight percentage relative to the total weight of the anhydrous zeolite adsorbent. In the description of the invention, the measurement uncertainty with respect to the molar ratio K ₂O/(K₂O+BaO+Na₂ O) is 0.2%.

Particle size of zeolite adsorbent:

The number average diameter of the zeolite adsorbent obtained at the end of the agglomeration and shaping step a) is determined by: the particle size distribution of the agglomerated samples was analyzed by imaging according to standard ISO 13322-2:2006 by passing the samples in front of a camera lens using a conveyor belt.

The number average diameter is then calculated from the particle size distribution by applying the standard ISO 9276-2:2001. In this document, the term "number average diameter" or "size" is used for zeolite agglomerates. For the size range of the agglomerates of the present invention, the precision is about 0.01mm.

Mechanical strength of zeolite adsorbent:

A technique for characterizing the representative mechanical crush strength of the adsorbent in a bed or reactor is a technique for characterizing bulk crush strength as described in Shell Method series SMS1471-74 (Determination of Bulk Crushing Strength of catalysts.compression-Sieve Method) in combination with a BCS Tester machine sold by Vinci Technologies company. The method, originally intended for characterizing 3-6mm catalysts, is based on the use of a 425 μm sieve which makes it possible in particular to separate the fines produced during crushing. The use of a 425 μm screen is still suitable for particles having a diameter of greater than 1.6mm, but must be modified according to the particle size of the zeolite adsorbent desired to be characterized. Standard ASTM D7084-04 (which also describes a method for measuring the bulk crush strength of a catalyst) (Determination of Bulk Crush Strength of CATALYSTS AND CATALYST CARRIERS) defines the mesh size of the screen to be used to be equal to half the diameter of the catalyst particles to be characterized. The method contemplates a preliminary step of screening a sample of the catalyst or adsorbent to be characterized. If a sample in an amount equal to 10% by weight passes through the grid, a smaller mesh size screen will be used.

The agglomerates of the invention (typically in the form of beads or extrudates) generally have a number average diameter or length, i.e., the largest dimension in the case of non-spherical agglomerates: between 0.2mm and 2mm, in particular between 0.2mm and 0.8mm and preferably between 0.2mm and 0.65 mm. Thus, a suitable screen that allows less than 10% by weight of the sample to pass through the grid during the preliminary screening step is used instead of the 425 μm screen mentioned in the standard Shell method SMS 1471-74.

The measurement scheme is as follows: a 20cm ³ agglomerated sorbent sample, previously sieved with a suitable screen and previously dried in an oven at 250 ℃ (instead of 300 ℃ as mentioned in standard Shell method SMS 1471-74) for at least 2 hours, was placed in a metal cylinder with a known internal cross section. An increasing force was applied to the sample stepwise via a piston through a bed of 5cm ³ steel balls to best distribute the force applied by the piston on the adsorbent agglomerates (for spherically shaped particles having diameters strictly less than 1.6mm, 2mm diameter beads were used). The fines obtained at the various pressure stages are separated by sieving (with a suitable sieve) and weighed.

The bulk crush strength was determined by the pressure in megapascals (MPa) at which the amount of accumulated fines passing through the screen was 0.5% by weight of the sample. This value is obtained by plotting on a graph the mass of fines obtained as a function of the force applied to the adsorbent bed and by interpolating to 0.5% mass of the cumulative fines. The mechanical pack crush strength is typically between several hundred kPa and several tens MPa and usually between 0.3MPa and 4 MPa. The accuracy is conventionally less than 0.1MPa.

Determination of zeolite fraction of zeolite adsorbent:

The nature and amount of the various zeolite fractions are determined by X-ray diffraction (known to the person skilled in the art by the abbreviation XRD). The analysis was performed on a bruker brand machine and the amount of zeolite fraction was then assessed using TOPAS software from bruker company.

Micropore volume:

The crystallinity of the agglomerates was also evaluated by: their micropore volume was measured and compared to the micropore volume of a suitable reference (100% crystalline zeolite, or theoretical zeolite, under the same cationic treatment conditions). The micropore volume is determined from the adsorption isotherm of a gas such as nitrogen at its liquefaction temperature. The zeolite adsorbent is degassed under vacuum (P < 6.7x10 ^-4 Pa) at a temperature between 300 ℃ and 450 ℃ for a period of 9 hours to 16 hours prior to adsorption. Measurements of nitrogen adsorption isotherms at 77K were then performed on an ASAP 2010M machine from Micromeritics, at least 35 measurement points were taken at a relative pressure ratio P/P ₀ between 0.002 and 1. The micropore volume was determined from the isotherms obtained according to Dubinin and Raduskevitch by applying standard ISO 15901-3:2007. The micropore volume estimated from Dubinin and Raduskevitch is expressed in cm ³ liquid adsorbate/gram adsorbent. Measurement uncertainty is ±0.003.

Loss on ignition of zeolite adsorbent:

The loss on ignition is determined in an oxidizing atmosphere by calcining the sample in air at a temperature of 950 ℃ ± 25 ℃ as described in standard NF EN 196 2 (month 4 2006). The standard deviation of measurement is less than 0.1%.

Liquid phase adsorption is characterized by breakthrough (break-through):

The technique used to characterize the liquid phase adsorption of molecules onto porous solids is the "breakthrough" technique described by Ruthven in "PRINCIPLES OF ADSORPTION AND ADSORPTION PROCESSES" (chapters 8 and 9, john Wiley & Sons, 1984), which defines breakthrough curve technique as a study of the response to the injection of one level of adsorbable component. Analysis of the average discharge time (first element) of the breakthrough curve gives information about the amount of adsorption and also makes it possible to evaluate the selectivity between the two adsorbable components, i.e. the separation factor. Injection of non-adsorbable components used as tracers is recommended for evaluation of non-selective volumes. Analysis of the dispersion of the breakthrough curve (second element) allows the equivalent height of the theoretical (tower) plate, which is a direct measure of the axial dispersion and mass transfer resistance of the system, to be evaluated based on representing the tower (column) through a limited number of hypothetical reactors (theoretical stages) that are ideally stirred.

Examples

Preparation of zeolite adsorbents

105G of kaolin (indicated as calcined counterpart) and 45g under the trade name are used according to the procedure described in patent application FR 2 999 098 (synthesis of example B)The colloidal silica sold (comprising 30% by weight of SiO ₂ and 0.5% of Na ₂ O) and an amount of water that allows extrusion of the mixture produced a homogeneous mixture and agglomerated 800g of zeolite NaX crystals. The extrudate was dried, crushed to collect pellets (gran) having a number average diameter equal to 0.5mm, and then calcined under a nitrogen stream at 550 ℃ for 2 hours.

200G of the obtained agglomerate was placed in a glass reactor equipped with a jacket adjusted to a temperature of 100.+ -. 1 ℃ and then 1.5L of an aqueous sodium hydroxide solution having a concentration of 2.5M was added and the reaction medium was left to stand and stir for a period of 4 hours.

The agglomerates are then washed with water in 3 successive washing operations, after which the reactor is emptied. The washing efficiency was ensured by measuring the final pH of the washing water between 10 and 10.5.

Example 1: cation exchange of barium and potassium

The sodium cations of the obtained agglomerates were exchanged with barium and potassium ions in 4 steps at 95 ℃ by a 0.5M aqueous solution of potassium chloride and barium chloride. The potassium chloride and barium chloride concentrations in the solution were modified to achieve the target barium and potassium content in the adsorbent and thus the target K ₂O/(K₂O+BaO+Na₂ O molar ratio (fig. 1). In particular, a molar ratio of K ₂O/(K₂O+BaO+Na₂ O) of 8.0% is achieved with an aqueous solution of barium chloride concentration of 0.40M and potassium chloride concentration of 0.10M. In each step, the ratio of the volume of solution to the mass of solid was 20mL/g and the exchange was continued for 3 hours each time. Between exchanges, the solid is washed several times so that it does not contain excess salt. The agglomerates were then dried at 80 ℃ for 2 hours and finally activated at 250 ℃ for 2 hours under a nitrogen stream.

For each sample, the loss on ignition measured as described previously was 5.6% ± 0.1%. The extent of barium+potassium exchange of the agglomerates, calculated by elemental analysis of barium oxide and sodium oxide by X-ray fluorescence as described in the characterization technique, was 99.7±0.2%. In particular, for K ₂O/(K₂O+BaO+Na₂ O) the molar ratio is 8.0%, the sodium oxide Na ₂ O content is 0.05% by weight relative to the total weight of the adsorbent, the barium oxide BaO content is 35.15% by weight relative to the total weight of the adsorbent, and the potassium oxide K ₂ O content is 1.89% by weight relative to the total weight of the adsorbent.

EXAMPLE 2 penetration test

The agglomerates obtained in example 1 were then subjected to a penetration test (head-on (frontal) chromatography) to evaluate their effectiveness. The amount of adsorbent used in this test was about 30g.

The procedure for obtaining the penetration curve is as follows:

fill column with screen and insert into test stand,

Filled with solvent (toluene) at room temperature.

Gradually increase to adsorption temperature under solvent flow (2 cm ³/min).

The solvent was injected at 2cm ³/min when the adsorption temperature was reached.

Solvent/feedstock substitution to inject feedstock (2 cm ³/min).

The injection of the feedstock is then maintained for a time sufficient to reach thermodynamic equilibrium.

The breakthrough retentate (retaining) was collected in a single flask, after which the composition of the retentate was analyzed by GC.

The pressure is sufficient to keep the feedstock in the liquid phase, i.e. 1MPa. The adsorption temperature was 175 ℃. The composition of the raw materials used for the test was as follows:

Para-xylene: 18% by weight

M-xylene: 18% by weight

Ortho-xylene: 18% by weight

Ethylbenzene: 18% by weight

P-diethylbenzene: 18% by weight

Isooctane: 10% by weight (this was used as a tracer for evaluation of non-selective volumes and did not participate in the separation)

The selectivity of the paired compounds (denoted as binary selectivities a _i/k) was calculated from the adsorption amounts q _i and q _k of compounds i and k, the latter being determined from analysis of the composition of the breakthrough retentate and the composition of the feed (feed in which the mass fractions of compounds i and k are y _i and y _k), by means of the feed balance:

Evaluation of the potential of these adsorbents during simulated countercurrent operation was based on equilibrium theory as described in Mazotti, storti and Morbidelli at month 11 of Robust Design of Countercurrent Adsorption Separation Processes:2.Multicomponent Systems,AIChE Journal,1994, volume 40, 11 applied to multicomponent systems with constant selectivity. In particular, reference is made here to equation 8, which describes that in order to obtain a complete separation of the 4 segments (j=1-4) of the countercurrent separation unit as schematically represented in fig. 1 of the cited paper, the conditions with respect to the reduced conveying rate m _j should be fulfilled.

Section 1: k _ss＜m₁δ₁ < ++ infinity

Section 2: k _wk＜m₂δ₂＜K_sk

Section 3: k _wk＜m₃δ₃＜K_sk (8)

Section 4:

This equation 8 mentions the adsorptivity K _i of the various components and also mentions the parameter delta _j for each segment j defined by equation 7:

It should be noted here that by definition the binary selectivity α _i/k between compounds i and K is equal to the ratio of the adsorptivity K _i/K_k.

The reduced transport rate of each segment of the unit is defined as the ratio of the transport rate of the liquid phase to the transport rate of the adsorbent phase. Equation 8 indicates what the limit reduced delivery rate is for each segment. In a 4-stage countercurrent separation unit, the feed transport rate corresponds to the difference between the transport rate in zone 3 and the transport rate in zone 2.

Thus, when it is desired to evaluate the maximum productivity achievable with a given adsorbent, what is sought is to evaluate the maximum amount of feedstock that can be processed, i.e., the difference between the maximum transport rate in zone 3 and the minimum transport rate in zone 2. Performance quality can be compared according to the maximum productivity of the two adsorbents by: comparing their reduced transport rates by zones 2 and 3 (m ₂ and m ₃, respectively) according to the relationship: max (m _{Raw materials})＝max(m₃)–min(m₂) determines the maximum reduced feed rate of the feedstock.

If a system with constant selectivity is considered, the composition of the liquid phase giving the greatest stress in zone 2 and in zone 3 is the composition of the liquid phase at the point where the feedstock is injected into the unit. In particular, from this point onwards, the concentration of para-xylene, which is the most adsorbed compound, increases in the direction of flow of the solid in zone 2 and decreases in the direction of flow of the liquid in zone 3. The composition of this point can be approximated as the composition of the feedstock to be treated, and it is this composition that will be used to evaluate the terms δ ₂ and δ ₃ of equation 8. The terms δ ₂ and δ ₃ are defined by equation 7 mentioned above. For each adsorbent, the reduced transport rate max (m _{Raw materials}) was calculated from the experimentally measured binary selectivity values and is represented in fig. 1 as a function of the molar ratio of K ₂O/(K₂O+BaO+Na₂ O (fig. 1). It is seen that for a molar ratio of K ₂O/(K₂O+BaO+Na₂ O between 8.0% and 8.6%, the reduced delivery rate max (m _{Raw materials}) is improved.

Claims

1. A zeolite adsorbent comprising zeolite X crystals and comprising barium, potassium and sodium, wherein K ₂O/(K₂O+BaO+Na₂ O) molar ratio is between 8.0% and 8.6%.

2. The adsorbent of claim 1, further comprising a non-zeolitic phase.

3. An adsorbent according to claim 1 or claim 2, wherein the content of sodium oxide Na ₂ O is less than 0.3% by weight relative to the total mass of the adsorbent.

4. The adsorbent according to any one of the preceding claims, wherein the total content of alkali or alkaline earth metal ion oxides other than barium oxide BaO, potassium oxide K ₂ O and sodium oxide Na ₂ O is less than 1% by weight relative to the total mass of the adsorbent.

5. The adsorbent of any one of the preceding claims, wherein the zeolite X crystals have a Si/Al atomic ratio between 1.00 and 1.50.

6. The adsorbent according to any one of the preceding claims, having a number average diameter of between 0.2mm and 2mm.

7. An adsorbent according to any one of the preceding claims, wherein the zeolite X crystals have a number average diameter of less than or equal to 1.5 μm.

8. The adsorbent according to any one of the preceding claims, having a loss on ignition of between 4.0% and 7.7% by weight measured at 950 ℃ according to standard NF EN 196-2.

9. The adsorbent according to any one of the preceding claims, wherein the mass fraction of zeolite X is at least 80% by weight relative to the total weight of the adsorbent.

10. A process for preparing an adsorbent according to any one of the preceding claims, comprising at least the steps of:

a) Agglomerating zeolite X crystals with a binder and shaping the agglomerates, followed by drying and calcining,

B) Optionally the binder is zeolitized,

C) Cation exchange of the agglomerates by contact with barium ions, or potassium ions, or a solution of barium and potassium ions,

E) The product thus treated is then washed and dried, and

F) The zeolite adsorbent thus obtained is activated.

11. The method according to claim 10, wherein the binder used in step a) comprises at least 80% by weight of a zeolitizable clay and a silica source, and wherein the method comprises a step b) of zeolitizing the zeolitizable binder via the action of an alkaline solution, preferably with a solution having a concentration between 0.5M and 5M and for a time between tens of minutes and hours.

12. A process according to claim 10 or claim 11, wherein zeolite X used in step a) comprises, and preferably is, a classified porous zeolite X-type FAU.

13. The adsorbent according to any one of claims 1-9, obtainable according to the process of any one of claims 10-12.

14. Use of an adsorbent according to any one of claims 1-9 or according to claim 13 in a process for:

separation of aromatic C8-isomer fractions and in particular xylenes,

The separation of the cresols,

Separating the polyol.

15. Use according to claim 14 for separating para-xylene from an aromatic isomer fraction having 8 carbon atoms.

16. Process for the recovery of para-xylene in the liquid phase from an isomer fraction of aromatic hydrocarbons containing 8 carbon atoms by adsorption of para-xylene using an adsorbent according to any of claims 1-9 or 13 in the presence of a desorbent, preferably selected from toluene and para-diethylbenzene.

17. The process for the recovery of para-xylene according to claim 16, which is of the simulated moving bed type, preferably using simulated countercurrent flow.

18. Process for the recovery of para-xylene in the gas phase from an isomer fraction of aromatic hydrocarbons containing 8 carbon atoms by adsorption of para-xylene using an adsorbent according to any of claims 1-9 or 13 in the presence of a desorbent, preferably selected from toluene and para-diethylbenzene.

19. A process for separating polyols using the adsorbent according to any one of claims 1 to 9 or 13.

20. A process for separating substituted toluene isomers such as nitrotoluene, diethyltoluene or toluenediamine using the adsorbents according to any one of claims 1 to 9 or 13.

21. A process for separating cresols using an adsorbent according to any one of claims 1 to 9 or 13.